The farms with SPM cases all had one thing in common: the presence of box elder trees “laden with seeds,” Valberg said.

Photo: Stephanie Valberg, DVM, PhD

A horse starts showing stiffness and a reluctance to move. His muscles suddenly become weak to the point he can no longer remain standing. Then, as quickly as clinical signs set in, the horse dies.

Just 48 hours earlier the horse grazed happily in his pasture—an overgrazed field full of seed heads and dead leaves.

This story is typical of suspected cases of seasonal pasture myopathy (SPM), a highly fatal muscle disease described in the Midwestern United States and eastern Canada, and atypical myopathy (AM) in the United Kingdom and Europe. For decades the disease had baffled veterinarians on both continents, who struggled to pinpoint and agree on a cause.

That changed in 2011 when Stephanie Valberg, DVM, Dipl. ACVIM, and a team of researchers from University of Minnesota (UM) and Iowa State University (ISU) started investigating SPM cases and found a link to box elder trees. She presented their findings in “Identification of the Cause of Seasonal Pasture Myopathy in Horses” at the 2014 American College of Veterinary Medicine Forum, held in Seattle, Wash.

Disease Overview and Clinical Signs

Valberg started her talk with an overview of the disease. SPM and AM outbreaks vary year to year and usually occur in the fall with a few cases in taking place the following spring, she said. In North America, only a few horses are affected on a given pasture, she said, pointing out that European outbreaks include many horses on the same farm.

“Horses that develop SPM and AM are usually kept on sparse pastures with an accumulation of dead leaves, dead wood, and trees in or around the pastures," Valberg explained.

Unlike other muscle disorders, the disease’s clinical signs are not associated with exercise and include:

Reluctance to move;

Muscle weakness;

Stiffness and fine muscle tremors;

Increased periods of recumbency (unable to rise after lying down);

Tachycardia (an irregular and overly rapid heartbeat);

Myoglobinuria (red-brown colored urine); and

Occasionally choke (esophageal obstruction).

These clinical signs progress quickly, Valberg said, with rapid respiratory rate within 48 hours and dyspnea (difficulty breathing) and death within 72 hours in at least 75% of cases. The cause of death is a very specific metabolic block in the muscle’s ability to burn fat for fuel, Valberg said.

The Study

Valberg and her team looked at 12 SPM cases that occurred at 11 Midwestern farms. Of the 12 cases, the team defined two as suspect and 10 as confirmed.

The team visited the farms located in Iowa and Minnesota, taking photos of pastures, plants, and trees. Team members also noted information about pasture topography, any overgrazing, and whether the pastures contained dead wood or standing water. The team obtained photos from farms included in the study located outside of those two states, Valberg explained.

A control group for the study included 23 Minnesota farms with no history of SPM in resident horses.

The farms with SPM cases all had one thing in common: the presence of box elder trees (belonging to the Acer or maple family) “laden with seeds,” Valberg said. Other commonalities? SPM horses at all the affected farms received 24-hour turnout, “which was significantly longer than horses on the control farms,” Valberg said, and all SPM pastures were overgrazed.

In contrast, the study found that control farms were less likely to have box elders (61% compared to 100%). Additionally, they were less likely to have overgrazed pastures (44% compared to 100%), and horses usually received additional hay in stalls or on pasture.

Horses at SPM farms were less likely to receive supplemental hay or concentrate compared to those at the control farms, Valberg said.

“Thus, the common features for SPM horses were little supplemental feeding and prolonged grazing of sparse pastures that were in close proximity to seed-laden box elder trees,” she said.

With the box elder as suspect No. 1, the research team collected box elder seed samples randomly from seven of the 11 SPM farms during fall 2011, as well as a control sample of ash tree seeds. Testing found that one amino acid, hypoglycin A, was “highly abundant” in the box elder seeds but not in ash seeds. This amino acid, also found in unripe fruit of the ackee tree (within the same tree family as box elder), is known to block the same specific enzyme in fat metabolism in humans as was seen in SPM-afflicted horses. In humans, eating unripe ackee fruit causes Jamaican vomiting disease.

Most recently, scientists have found another related tree, the European sycamore maple tree (Acer pseudoplatanus), in northern European pastures where horses have died from AM, Valberg said. European sycamore maple seeds also contain hypoglycin A.

“Hypoglycin A is a common cause of both SPM and AM,” Valberg concluded. “This is supported by common epidemiology of SPM and AM, a common block on fat metabolism, the presence of hypoglycin A containing Acer species in affected pastures, and the presence of the toxic principle of hypoglycin A in the bloodstream of afflicted horses.”

Conclusions: Protecting Your Horses

Valberg noted that some horses do not develop SPM after years of living on affected pastures. “This can make discussions with owners regarding prevention complicated,” she said.

Young horses and those new to an affected pasture appear to be at great risk if:

Pastures are overgrazed in the fall and early spring;

Turnout time is greater than 12 hours per day; and

No supplemental hay is provided on pasture.

To address potential concerns of SPM developing in horses, Valberg said removal of the box elder or European sycamore trees from affected pastures can be one approach, although, “this might not always be feasible.”

“While box elders rarely live longer than 50 years, some of the sycamore maples on affected pastures are huge beautiful trees that owners may be very reluctant to remove,” she explained.

In cases where the trees can’t be removed, Valberg recommended decreasing turnout time on affected pastures from October through mid-December and in the early spring.

“Other important preventive measures could include providing additional forage if pastures are overgrazed, preventing over grazing of pastures through rotational grazing, and limiting turnout to less than 12 hours per day during fall and early spring,” she said.

More research needs to be done to determine if seasonal and annual fluctuations occur in box elder and sycamore maple trees' hypoglycin A content, she concluded.

About the Author

Michelle Anderson serves as The Horse's digital managing editor. In her role, she produces content for our web site and hosts our live events, including Ask the Vet Live. A lifelong horse owner, Anderson competes in dressage and enjoys trail riding. She's a Washington State University graduate (Go Cougs!) and holds a bachelor's degree in communications with a minor in business administration and extensive coursework in animal sciences. She has worked in equine publishing since 1998. She currently lives with her husband on a small horse property in Central Oregon.

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